import的疑问

2010-03-19 11:28:50 118 阅读 AI 写同款 · GEO 优化AI 选题数据监测一键分发 ›

Python import 和 from … import 的主要区别适用于需要使用整个模块，避免命名冲突，提高代码可读性。：适用于只需要模块中部分对象的情况，简化代码，但需注意命名冲突。：尽量避免使用，可能导致命名冲突和代码混淆。选择适合的导入方式取决于具体的使用场景和需求，通常推荐在需要避免命名冲突和保持代码清晰时使用import语句，在需要简化代码和使用特定对象时使用语句。阅读详情

说明：下面有三段代码，实际上后两段代码是将第一段代码拆分成两个java文件保存了，实现功能完全一样
问题：就我的理解，import java.util.*的作用是引入util下所有的类库，但是在第一段代码中，我还必须专门引入Date，否则编译就会报错，但如果像后两段代码一样，拆分开的，不用再单独的引入Date，一样可以编译成功，这是为什么？谢谢！

代码1：



import java.util.Date;

import java.util.*;



public class EmployeeTest

{

	public static void main(String[] args)

	{

		Employee[] staff = new Employee[3];

		

		staff[0] = new Employee("Simon Lo",2450,2006,8,25);

		staff[1] = new Employee("Seven Huang",3500,2006,06,01);

		staff[2] = new Employee("Eric Yap",5000,2000,01,15);



	

		for (Employee e : staff)

			e.raiseSalary(5);

	

		for (Employee e : staff)

			System.out.printf("\nName=" + e.getName() + "\nSalary=" + e.getSalary() + "\nHireDay=" + e.getHireDay());

	}

}



class Employee

{

	public Employee(String n, double s, int year, int month, int day)

	{

		name = n;

		salary = s;

		GregorianCalendar calendar = new GregorianCalendar(year, month -1, day);

		hireDay = calendar.getTime();

	}

	

	public String getName()

	{

		return name;

	}

	

	public double getSalary()

	{

		return salary;

	}

	

	public Date getHireDay()

	{

		return hireDay;

	}

	

	public void raiseSalary(double byPercent)

	{

		double raise = salary * byPercent / 100;

		salary += raise;

	}

	

	private String name;

	private double salary;

	private Date hireDay;

}

代码2：



import java.util.*;

class Employee

{

	private String name;

	private double salary;

	private Date hireDay;

	

	public Employee(String n, double s, int year, int month, int day)

	{

		name  = n;

		salary = s;

		GregorianCalendar calendar = new GregorianCalendar(year,month - 1,day);

		hireDay = calendar.getTime();

	}

	

	public String getName()

	{

		return name;

	}

	public double getSalary()

	{

		return salary;

	}

	public Date getHireDay()

	{

		return hireDay;

	}

	

	public void raiseSalary(double byPercent)

	{

		double raise = salary * byPercent /100;

		salary = salary + raise;

	}

	

	

	//下面的main方法是用来测试Employee类的，并不会在整个程序中执行

	public static void main(String args[])

	{

		Employee test = new Employee("Simon Lo",3000,2006,8,1);

		test.raiseSalary(10);

		System.out.println("There is some Unit Test blow this line:");

		System.out.printf(test.getName() + "\t" + test.getSalary());

	}



}

代码3：



import java.util.*;



public class EmployeeTest

{

	public static void main(String args[])

	{

		Employee[] staff = new Employee[3];

		

		staff[0] = new Employee("Carl Cracker", 75000, 1987, 12, 15);

		staff[1] = new Employee("Harry Hacker", 50000, 1989, 10, 1);

		staff[2] = new Employee("Tony Tester", 40000, 1990, 3, 15);

		

		for (Employee e : staff)

			e.raiseSalary(5);

		

		for (Employee e : staff)

			System.out.printf("Name=" + e.getName() + "\tsalary=" + e.getSalary() + "\thireDay=" + e.getHireDay() + "\n");

	}

}